Antihunt electrical servomotor system



Sept. 2, 1947.

W. L. SHAFFER AN'I'IHUNT ELECTRICAL SERVOMOTOR 5 Filed D66. 19, 1942 FIGI.

YSTEM 2 SheetsSheet l WILLIAM L. SHAFFERv Sept. 2, 1947.

w. L SHAFF ER ANTIHUNT ELECTRICAL SE RVOIIOTOR SYSTEM Filed Dec. 19, 1942 2 Sheets-Sheet 2 FIG. 2.

I43 v ms I r -N56 INVENTOR. WILLIAM L. SHAFFER.

ATTORNEY.

Patented Sept. 2, 1947 ANTIHUNT ELECTRICAL SERVOMOTOR SYSTEM William L. Shaffer, Philadelphia, Pa., assignor to The Brown Instrument Company, Philadelphia, Pa., a corporation of Eennsylvania Application December 19, 1942, Serial No. 469,533

6 Claims. 1

The present invention relates to electrical apparatus of the self-balancing type for ascertaining the magnitude or changes in magnitude of a variable condition such as temperature, pressure, flow, liquid level, etc. and including suitable provisions for indicating or making a record of the changes and/or maintaining the condition at a predetermined value.

A general object of the invention is to provide improved anti-hunting means in such indicating, recording and/or controlling apparatus.

A more specific object of the invention is to provide improved stationary anti-hunting means in such apparatus.

The provision of suitable anti-hunting means in self-balancing recording and/or controlling systems is desirable since the inertia of the various mechanically connected parts comprising the driving structure tends to so operate as to effect rebalancing adjustment beyond the proper position of rebalance. When such a condition of overshooting prevails, the system then becomes unbalanced in the opposite direction and initiates operation oi the driving structure in the reverse direction to enect'rebalance, but again the necessary regulation is exceeded to thereby set up a continuous hunting of the driving structure about the balance point.

The need for eliminating hunting in measuring and controlling apparatus has long been recognized and various arrangements have been proposed to attain this end. Thus, according to one prior art arrangement this result is accomplished by operating the rebalancing driving structure in accordance with the extent of unbalance of the system, and by providing mechanicall movi means for anticipating the approach of the ng structure to the true balance position. ,other prior art arrangement means are proed in the form of a generator actuated by the ig structure for obtaining an electromotive force proportional to the speed of operation of the driving structure. This electromotive force is introduced into the measuring or detector circuit 1 opposition to the unbalanced electrometiv'e iorce of the condition responsive network and operates to produce a condition of simulated rebalance of the network before the latter is actually rebalanced to thereby anticipate the true balance position of the network.

It is a specific object of the present invention to provide new and improved anti-hunting means for use in measuring and controlling apparatus in which an electromotive force proportional to the extent of unbalance of the condition responsive network is obtained without requiring the use of any physically movable means additional to the rebalancing structure, and in which simple and emcient means under control oi the derived electromotive force are provided to anticipate the approach of the driving structure to the balance point and thus neutralize the tendency of the system to hunt.

further and more specific object of the in vention is to provide in self-balancing measuring and/or controlling apparatus, utilizing electronic amplifier means, a variable-mu tube the amplification factor of which is varied under control of the derived electromotive force to minimize hunting of the apparatus,

The various features oi novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part or this specification. For a better understanding of the invention, however, its advantages and specific objects attained with its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodiment of the invention.

Of the drawings: 7

Fig. l is a diagrammatic representation of the use of one embodiment of my invention in a temperature control system in which an electrical bridge network is utilized to detect the temperature variations;

Fig. 2 illustrates the use of my invention in a potentiometric type temperature recording and controlling system; and

Fig. 3 illustrates the use of still another embodiment of my invention.

Referring to Fig. 1, my invention is shown, by way of example, as applied to a control system wherein the position of a valve 4 is modulated in accordance with the balance or unbalance of a bridge network 2 which includes a temperature responsive resistance 3 in one arm. The temperature responsive resistance 3 is disposed in a room or space 4 which is heated by a furnace supplied with fuel through valve I. The unbal- 3 anced potential of the bridge network 2 is applied to the input circuit of an electronic amplifier having two preliminary amplifying stages and 8 and an output stage I. A pair of relays 8 and 8 are selectively operated by the output stage I to control the rotation and direction of rotation of reversible electrical motor ID. The valve is actuated through a gear train II nd a. rack H by motor l8. Motor I8 is provided with a pair of windings |3 and I4 whose selective energization causes rotation of the motor in one direction or the other. Electric power is supplied to the system from a suitable source of alternating current which has been designated by the reference character l5 and to which is connected the primary winding |8 of a transformer ll. Transformer H has a plurality of secondary windings |8, I9, 28, 2|, 22 and 23. Winding |8 supplies power to the bridge network 2, windings I9, 28, 2| and 22 supply power to the amplifying stages, and winding 23 supplies power to the motor windings I3 and I4.

The motor I8 is of the conventional induction type and is provided with two rotors 24 and 25 which are axially displaced on the motor shaft.

Field winding I3 is individual to the rotor 24 and field winding I4 is individual to the rotor 25. When the motor is operating with the field winding l3 energized, the direction of rotation is such as to increase the opening of the valve I and thereby to raise the temperature of the space 4. Conversely, the field winding |4 when energized produces opposite rotation so as to reduce the valve opening and lower the temperature of space 4.

The bridge network 2 has a pair of input terminals 28 and 21 and a pair of output terminals 28 and 29. Output terminal 29 is the point of engagement of a slider contact arm 38 on a slidewire resistance 3|. Input terminals 28 and 21 are connected to the terminals of the transformer secondary winding I8 by means of conductors 32 and 33.

Bridge network 2 includes the temperature responsive resistance 3 in the upper left arm and includes a fixed resistance 34 and a variable resistance 35 in the upper right arm. The lower left arm includes a fixed resistance 38 and a portion of the slidewire resistance 3| depending upon the position of the slider contact arm 30. The lower right arm includes a fixed resistance 31 and the remainin portion of the slidewire resistance 3|. The adjustable resistance 35 is provided so that the position of valve to be maintained in response to a iven temperature adjacent resistance 3 may be manually determined.

Output terminal 28 of bridge network 2 is connected by means of a conductor 38 to the control electrode 39 of a variable-mu pentode 48 which also includes an anode 4|, a suppressor grid 42, a screen grid 43 and a cathode 44 and forms a part of the first preliminary stage 5 of the electronic amplifier. The other output terminal 28 of bridge network 2 is connected to the cathode 44 of pentode 48 through a circuit which may be traced from the terminal 28 through slide contact arm 38, a conductor 45, a contact 48 which is in engagement with a slidewire resistance 41, a conductor 48, and a biasing resistance 49 which is shunted by a condenser 58 to the cathode 44 of pentode 48. The conductor 48 is connected to ground as indicated at G. A resistance 5| of suitable value is connected between the bridge network output terminals 28 and 29.

Pentode 48 comprises part of a triode pen'tode tube 52 which may be of the type sold commercially as the type BF'I. Tube 52 includes a triode 53 in addition to the pentod 48, which triode forms a part of the second preliminary stage 8 of the amplifier. The cathode 44 is common to the pentode 48 and triode 53. Triode 53 also includes an anode 54, and a control grid 55. A heater filament 44a which has its terminals connected to the transformer secondary winding I8 is provided for heating the cathode 44.

The grounded conductor 48 forms the negative terminal of the power supply for the two preliminary amplifying stages 5 and 8. Conductor 58 forms the positive terminal of the power supply for these stages. A unidirectional potential is supplied between conductors 58 and 48 by means of a rectifier circuit connected to the transformer secondary winding 2|. This rectifier circuit includes a diode 51 having an anode 58, a cathode 59 and a heater filament 88. Energizing current is supplied to the heater filament 88 from the transformer secondary winding 28.

The rectifier circuit may be traced from the left end terminal of transformer secondary winding 2| to the anode 58 of diode 51, and the cathode 58 I to the positive terminal of a filter condenser 8 I, to which terminal conductor 58 is also connected. The other and negative terminal of condenser 8| is connected to the grounded conductor 48 and therethrough to the right end terminal of the transformer secondary winding 2|. Thus, conductor 58 is connected to the positive terminal of condenser 8| and conductor 48 is connected to the negative terminal of condenser 8|.

The output circuit of the first preliminary amplifying stage 5 may be traced from the positive conductor 58 through a load resistor 82, anode 4|, cathode 44, and the parallel connected resistance 49 and condenser 58 to the grounded negative conductor 48.. The suppressor grid 42 and the cathode 44 of pentode 48 are internally connected. Screen grid voltage is obtained for pentode 48 by connecting the screen grid through a resistor 83 to the positive conductor 58 and by connecting the screen grid through a condenser 84 to the grounded negative conductor 48.

The output circuit of preliminary amplifying 'stage 5 is coupled to the input circuit of the second preliminary amplifying stage 8 by a con denser 85 one terminal of which is connected by aconductor to the anode 4| and the other terminal of which is connected to control grid 55 of triode 53. Control grid 55 is also connected through a resistance 88 to the negative conductor 48 and through the parallel connected elements 49 and 58 to the cathode 44. The condenser 85 operates to transmit any alternating or fluctuating potential appearing in the output circuit of stage 5 to the input circuit of stage 8 while simultaneously blocking the transmission of any direct current potential from the output circuit of stage 5 to the input circuit of stage 8.

The output circuit of preliminary amplifying stage 8 may be traced from the positive conductor 58 through a load resistor 81, anode 54, cathode 44, and parallel connected resistance 48 and condenser 58 to the negative conductor 48, and is coupled to the input circuits of the amplifier output stage I.

Output stage 1 includes a pair of triodes 88 and 89 which ma be contained within the same bulb, as shown. Triode 88 includes an'anode I8, 9. control grid 1|, a cathode l2 and a heater filament 13, Similarly, triode 88 includes an anode 14, a control grid 15, a cathode l8 and a heater filament 11. Heater filaments l3 and 11 have their terminals connectedto the terminals or th transformer secondary winding l9 and receive energizing current from the latter winding.

The input circuits of triodes 68 and 69 of stage I are connected in parallel and are coupled by a condenser 18 and a resistance 19 to the output circuit of the preliminary amplifyin stage 6. To this end one terminal of condenser 18 is connected to the anode 54 of triode 53 and the other terminal of condenser 18 is connected to the control grids II and 15 of triodes 58 and 69, which, as shown, are connected together. The control grids H and 15 are connected by the resistance 79 to the grounded negative conductor 49 which, in turn is connected to a center tap 89 provided on the transformer secondary winding 22. A tap 8! on the winding 22 to the left of center tap 80 is connected by a conductor 82 to the left cathode '12 of triode 58, and a tap 83 on the winding 22 to the right of center tap 99 is connected by a conductor M to the cathode 3:5. The alternating potential produced between the taps 89 and Bi serves to bias the control grid ll of triode 88 negatively with respect to the cathode it while the alternating potential produced between the taps 89 and 83 operates to bias the control grid '15 of triode 69 negatively with respect to the cathode 16.

The output circuit of preliminary amplifying stage 6 is also coupled by means of condenser 78 to the resistance 41 previously referred to as having at least a portion thereof included in the input circuit of pentode 49. This coupling circuit includes a triode rectifier 85 having an anode 96, a control grid 81, a cathode B8 and a heater filament 89. Energizing current is supplied the heater filament 89 from the transformer secondary winding 29. The control grid 87 and anode 86 are directly connected to each other and to the terminal of resistance 41 which is remote from the grounded negative conductor 48. The cathode 88 is connected by the condenser 18 to the anode 54 of triode 53.

In this manner the alternating or fluctuating potential of the output circuit of preliminary stage 6 and appearing between the anode 54 of triode 53 and the grounded negative conductor 49 is rectified by the rectifier B5 to produce a unidi-' rectional potential drop across the resistance l? having a magnitude corresponding to the magnitude of the fluctuating potential of the output circuit of stage 6 and of a given polarity irrespective of the phase of that fluctuating potential. A condenser 99 is connected in parallel to the resistance ll for the purpose of smoothing out the pulsations in the unidirectional potential drop produced across resistance 41. This unidirectional potential drop so established across resistance d! is utilized in a manner explained in detail hereinafter to neutralize the hunting tendency of the system.

Anode voltage is supplied the output circuits of the triodes 58 and 69 from the transformer secondary winding 22. Thus, the anode circuit of triode 98 may be traced from the left end terminal of the winding 22 through a, conductor 9i, in

which the operating winding 92 of relay 9 is inserted, to the anode, "l0, and cathode 72 to the tap 8| on the winding 22. The anode circuit of triode 69 may be traced from the right end terminal of the transformer secondary winding 22 through a conductor 93, in which the operating winding 94 of relay 5 i inserted to the anode l9, and cathode 15 to the tap 83 on winding 22.

Relay 8 controls the operation of a switch arm 95 into and out of engagement with a fixed contact 96. Switch arm 95 is biased by spring or other means away from contact 96. When switch arm 95 engages contact 96. anenergizing circuit is completed for the field winding l3 of reversible motor ID. This circuit may be traced from the left end terminal of the transformer secondary winding 23 through a, conductor 91 to the switch arm 95, contact 96, conductor 98, motor field winding I3, and a conductor 99 to the right end terminal of the winding 23.

Relay 9 controls the operation of a switch arm I09 into and out of engagement with a fixed contact llll. Switch arm I 00 is biased by spring or other means away from contact 10!. When switch arm I99 engages contact ii, an energizing circuit is completed for the field winding 14 of motor I9, This circuit may be traced from the left end terminal of transformer secondary winding 23 through conductor 97, switch arm,

J99, contact it i, conductor 192, motor field winding it, and conductor 99 to the right end terminal of winding 23. 0

If desired, suitable limit switches may be connected in the energizing circuits for the motor field windings i3 and it to interrupt the energizetion of motor 1 9 for rotation when the latter has operated to the extreme limits of adjustment of the fuel valve i. Such limit switches may be operated in any convenient manner by a suitable member connected to the shaft of motor 19.

When the bridge network 2 is balanced, the output terminals 28 and 29 are at the same potential, and therefore, no signal is applied to the in-- put circuit of the amplifier. Accordingly, no signal is produced in the output circuit of the second preliminary stage 6 and no signal is applied to the input circuits of the output amplifier stage i of which triodes 68 and 69 comprise a part. Under this condition triodes 68 and 69 are equally conductive but due to the bias potential impressed on the input circuits thereof by the portion of transformer secondary winding 22 between the taps 89, 93 and 89, 83 the current flow through each of the relay operating windings 92 and .94 will be below the operating level of the relays 9 and 9 and both of the switch arms will then be held out of engagement with their associated contacts. Motor H3 is then deenergized, and therefore, the valve l is maintained in the position to which it has been previously adjusted.

Upon a decrease in the temperature of the room or space 3 in which the resistance 3 is positioned, an alternatingpotential of predetermined phase relatively to the alternating potential at the terminals of transformer secondary winding 22 and impressed on the anode circuits of triodes 69 and 69 appears between, the output terminals 28 and 29 ofthe bridge network i. This alternating potential at the terminals 28 and 29 of the bridge network is amplified by the preliminary amplifying stages 5 and 6 and is applied to the input circuits of the triodes 98 and 89 without any substantial shift in phase. Since this amplified potential is in phase with the terminal voltage of the transformer secondary winding 22, it is then in phase with the alternating potential applied from the portion of winding 22 to the right of tap 83 to the anode circuit of triode B9, and out of phase with the alternating. potential applied from the portion of winding 22 to the left of tap iii to the triode 98. As a result, the conductivity of triode 69 is increased to effect energization of relay it and operation of slide contact resistance 3| and simultaneously to increase the opening of valve I so that more heat is supplied to the room or space 4 to the temperature of which the resistance element 3 is subl when a. rise in temperature of resistance 3 t ke place, the alternating potential appearin at the terminals 28 and 29 is of the opposite phase from the case previously considered, and consequently the conductivity of triode 6B is increased to cause operation of relay 9 and thereby energization of motor ill for rotation in the direction to drive slide contact arm 30 to the right along slidewire resistance 31 to rebalance the bridge network 2, and to simultaneously adjust valve I toward its closed position, thereby reducing the supply of heat to the room or space 4.

If desired, the impedance of relay windings 92 and 94 may be so designed and the bias potentials on the input circuits of triodes 65 and 69 so chosen that both relays attract their contact arms when the bridge network is balanced. With such modification, the two motor field windings I3 and I4 are energized and oppose each other under balanced conditions, resulting in it normally stalled motor which provides a braking action and thus aids in preventing overrunning of the motor beyond the true position of balance of the slide contact arm 30 along slidewire resistance 3 I In order to positively ensure that the motor ill will not overrun the true balance position. of slide contact arm 30 along slidewire resistance 31 and thus establish an oscillating or hunting condition of the system, there is introduced into the input circuit of the preamplifying stage 5 a unidirectional potential of varying magnitude depending upon the magnitude of the unbalanced alternating potential which appears between the bridge network output terminals 28 and 29. This unidirectional potential is introduced in the input circuit of the pentode 40 in series with the unbalanced alternating potential established between the bridge terminals" and 29 and is derived from the output circuit of the preamplifying stage 6 by means of the rectifier triode 8-5, resistance 41 and condenser 90 in the manner previously explained herein. Specifically, the unidirectional potential referred to is that tapped off the slidewire resistance 41 between the lower end of resistance 41 and the slide contact 46.

The desired neutralization of the coasting and overrunning tendency of the system is accomplished by this unidirectional potential by virtue of the operation of the pentode 40 which, as previously noted, is a variable-mu pentode. The amplification factor of such electronic valves is a function of the negative bias potential applied to the control electrode, and in particular, decreases as the negative bias is increased. The cathode biasing resistance 49 and condenser 50 operate normally to maintain a predetermined negative bias on the control electrode of pentode 40, and accordingly, when no unidirectional potential appears across resistance 41 as is the case when the bridge network 2 is balanced, the amplification factor of pentode 40 is a predetermined value.

At this amplification factor the application of an alternating potential of given value on theinput circuit of pentode 40 from thebridge output terminals 28 and 29 as a result of bridge unbalance produces an alternating potential of predetermined magnitude in the output circuit of the preamplifying stage 6. This amplified alternating potential i impressed on the input circuits of the amplifier output stage 1 and thus controls the selective actuation of one relay I or 9 and thereby operation of motor M for rotation in one direction or the other. The current flow established by this amplified alternating potential in the resistance 41 is simultaneously rectified by the rectifier triode so that a unidirectional potential drop is produced across resistance 41 having a magnitude determined by the magnitude of the amplified alternating potential. The polarity of the unidirectional potential so established across resistance 41 remains the same irrespective of the phase of the alternating potential which creates it and is of the proper polarity to increase the negative bias impressed on the control electrode of pentode 40. Consequently, as a result of the application of an alternating potential on the input circuit of pentode 40 from the bridge circuit 2 the amplification of the pentode 40 is decreased and such decrease in amplification is effected regardless of the phase of the applied alternating potential and thereby regardless of the direction of bridge network unbalance.

It is noted that the use of condenser for smoothing out the unidirectional potential drop produced across resistance 41 causes some delay in the production or a potential drop across resistance 41 following the appearance of an alternating potential in the output circuit of the preliminary amplifying stage 6. This delay has the effect of permitting the full action of the bridge unbalanced potential to be utilized initially to establish operation of the motor III for rotation in one direction or the other. It may be desirable in some cases for there to be such a time lag between the time that the bridge unbalanced potential is impressed on the input circuit of the pentode 40 and the time that the amplification changing bias is impressed on that input circuit.

With this arrangement, therefore, the amplification of the pentode 40 is a maximum when the bridge network 2 is first unbalanced so that the full unbalanced effect is initially utilized to operate the relay 8 and 9 and thereby motor Ill.

As soon as a unidirectional bias is established across resistance 41 and is impressed on the input circuit of pentode 40, however, the amplification of the pentode 40 is decreased by an amount corresponding to the magnitude of the bridge unbalanced potential within limits depending upon the overload characteristics of the preamplifier stages 5 and 6.

Such decrease in amplification of pentode 40 has the same effect on the alternating potential applied to the amplifier output stage 1 as a predetermined adjustment of slide contact arm 30 along slidewire resistance 3| toward the new position of balance, and as a consequence, the motor I0 is deenergized before the slide contact arm 30 has reached the new. position of balance. This anticipating effect is accentuated due to the tendency of the condenser 90 to resist any change in potential drop thereacross and to maintain the potential thereacross even when the contact arm approaches the balance point. That is to say. upon unbalance of the bridge network 2 and the establishment of a unidirectional potential drop of corresponding magnitude thereacross, the motor III will operate to adjust'the slide contact arm 40 toward the new balanced position along slidewire resistance 3|. As the slide contact arm 30 approaches the point of balance of the bridge network the alternating potential impressed on the input circuit ofpentode 40 diminishes in value, but the unidirectional bias potential obtained from resistance 41 does not decrease in proportion due to the action of condenser 90 in tending to maintain the then existing potential. Consequently, a disproportionately large unidirectional bias potential is impressed on the input circuit of pentode 40 as the slide contact arm 30,

nears the new balance point with the result that the motor I is deenergized before the balance point is reached.

By adjusting the magnitude of the unidirectional feed back bias potential to pentode 40 and by properly delaying the change of this potential in response to change in the state of balance of bridge network 2 the motor I0 may be deenergized for rotation at the proper moment before the balance point of bridge network 2 is approached as is required to exactly neutralize the tendency of the motor I0 and its associated drive mechanism tooverrun or coast beyond the true balance point.

In Fig. 2 I have illustrated, more or less diagrammatically, the adaptation of my invention to a potentiometric recording and controlling system. More specifically, an arrangement is illustrated in Fig. 2 for recording and controlling the temperature of a furnace I03 in the interior of which a thermocouple I04 is arranged which is' responsive to slight changes in the temperature of the furnace. The thermocouple which may be located at a distance from the remainder of the potentiometric measuring circuit has its terminals connected by a pair of conductors I05 and I06 to the terminals of a potentiometric measuring cir-- cuit I08 which preferably is of the null type. The potentiometric circuit I08 includes a slidewire resistance I09 and an associated contact IIO which is adapted to be moved along the length of the slidewire I09. It will be understood that the potentiometric circuit I08 may be of any suitable type such as the Brown potentiometric type disclosed in Patent 2,150,502 issued to T. R. Harrison, E. H. Grauel and J. E. Kessler on March 14, 1939. The movable contact I I0 is attached to a suitable carrier which may be in the form of an internally threaded nut III adapted to ride on a screw threaded rod I I I which is rotated in one direction or the other under control of the thermocouple I04. A suitable reversible electrical motor generally designated b the reference numeral H2 is mechanically coupled in any convenient manner to the screw threaded rod Hi to rotate the latter at the desired speed and in the desired direction to thereby move the contact H0 along slidewire resistance I09 to rebalance the potentiometric measuring circuit I08 whenever the latter is unbalanced. The contact IIO is-a bridgin contact connecting the point H3 of the slidewire resistance IIO to a corresponding point of a slidewire resistance II4 which is arranged alongside the resistance I09. The resistance II4 has one end connected by the conductor I05 to one terminal of the thermocouple I04 and is employed for the purpose of avoiding measurement inaccuracies due to variations in resistance to the flow of current generated by the thermocouple I04 which would otherwise result from variations in the relative resistances of the portions of the resistance I09 at opposite sides of the point H3. The other terminal of the thermocouple I04 is connected by means of the conductor I06 to the center tap on the primary winding II5 of a transformer H6 having a secondary winding H1. One end terminal of the primary winding I l 5 is connected to a contact H8 of a current interrupting device designated generally at H9 and the other end terminal of the primary winding I I5 is connected to a contact I20 of the device H9. The current interrupting device may be of the type disclosed in the copending F. W. Side application, Serial No, 421,176 filed December 1, 1941, which issued on July 8, 1947 as Patent No. 2,423,524, and includes a vibrating reed HI and an operating winding I32 in addition to the contacts I20 and H8. The vibratin reed is positioned between the contacts I20 and H8 and is adapted to engage the latter contacts in alternation. The vibrating reed I2I is connected by a conductor I22 to the point I23 of the potentiometric measuring circuit I08.

The flow and direction of flow of current through the circuit branch from the point I23 of the potentiometric measuring circuit to the current interrupting device II9, the transformer I I6, the thermocouple I04, and the bridging contact I I0 to the point II3 depends upon the relation between the electromotive force produced by the thermocouple I04 and the potential difference between the potentiometric circuit points H3 and I23. The thermocouple I 04 is so connected to the potentiometric circuit that the electromotive force of the thermocouple opposes the potential difference between the points I I3 and E23. Movement of the contact IIO to the left and right operates to respectively decrease and increase the potential difference between the points H3 and I23. With suitable adjustments of the contact III), the potential diflerence between the points H3 and I23 will be equal and opposite to the electromotive force produced by the thermocouple I04 and no current will flow through the above mentioned circuit branch including the device H9, transformer III; and thermocouple I04. On an increase in the thermocouple electromotive force above the potential difierence between the points H3 and I23 current will flow in one direction through the interrupter H9 and the transformer primary winding H5 and such current flow may be eliminated by a suitable adjustment of the contact III! to the right. Conversely, when the electromotive force of the thermocouple fall below the potential difference between the potentiometric circuit points H3 and I23, the resultant current fiow through the interrupting device IIS: and the transformer primary I I5 will be in such a direction as to be eliminated by a suitable adjustment of the contact II 0 to the left.

The bridging contact H0 is adjusted along the slidewire resistances I09 and i I4 by the operation of the reversible electrical motor H2 which has its rotor mechanically connected to the threaded shaft III on which the nut IiI' which supports the bridging contact H0 is carried. The reversible electrical motor II2 has a pair of terminals vhich are connected to the output circuit of the amplifier output stage I and also has a pair of terminals which are connected to the alternating current supply source I5 through a suitable condenser I24.

For its intended use the motor II2 may be of the type diagrammatically shown in the drawing and comprising a rotor I25 and two pairs of oppositely disposedfield poles (not shown) on one fication hi the electronic amplifier to the input terminals of which the transformer secondary winding i ii is connected.

The electronic amplifier comprises two preliminar, airi piifrwiifig stages and 6 and the output lug stage i. all of which stages rerg current from the transformer tood that additional premay he provided, if tilt includes secondary lid in addition to the also includes a, prito has its te minals ooh I; suppl'gt source it.

iii per oi? which a winding lid is wound and on the pair or which a winding lltll is wound. .r ding HE is supplied with ehergisiug current ore the alternating current supply source Hi "ough condenser tilt. Due to the action of ti ei er tilt the cu ht which flows through winding lit, mil he in phase with the 'P' source.

it"! is amplified nary amplifying ms etlfected tiug stage oi ion oi? the rotor l?arts oi the humor als. stage 5 "he input urinals oi" the transformer "e connected in series "nee 41 tapped off by t circuit of 'oentode one]. energizing poenergized conductor t. conductor 48. A uninod hetween the circuit inciud manner,

, secondary winding I39 and l. electrical. mot it, cont riser Si is connected t shot 5 30M, lit

" out the r,

prellu'lit ary amplifying stage ncludes the triode 53. Ener' eutial is supplied the output circuit of i" our ti. Bi luctors 515 and 48. The l tit-lode it is coupled by means e condenser ill to the resistance t! through Tie rectifier ml to establish a unidirectional itial across resistance 4'! whenever an alter- 1 mg or fiuctuat 1g potential appears in the output circuit of triode 53v and is also coupled by means of the condenser 18 to the input circuit of the motor driving stage "i".

Motor driving stage i includes a tube 142 which contains two heater type triodes, designated by the reference characters I43 and M4, and within the same envelope. Both of the triodes Hi3 and I include anode, control electrode, cathode and heater filament elements which in triode l 43 have been designated by numerals M5, lit, MT and M8, respectively, and in the triode 1M have been designated by the numerals 9, I50, 15! and I52, respectively. The heater filaments I48 and IE2 are connected in parallel and have their terminals connected to the terminals of the transformer secondary winding I31 which also supplies energizing current to the heater filament 44a of tube 52 and heater filament 89 of tube 85. Transformer secondary winding MI is connected to and supplies energizing current to the heater filament of rectifier tube 51.

J contacts he v tilt of a transformer tilt try conduct l tii and is supplied it uh alternating cu :ei'lt thereirom. A permanent magnet it'l asst ited. with the vihratiug reed lift DIOt'i 'ZlL for polarizing and synchronizing purposes. r permanent ig in co-- J secondary winding I33. In consequence, unbalanced no tehtiometric currents are caused to alternately flow through the opposite halves of the trans" former primary winding l to in. such manner as to create an alternating potential in the tra sformer secondary winding H! which is of the same frequency as the alternating voltage supplied by the transformer secondary winding I33 and is in phase with-or displaced 180 in phase from the voltage of winding I33 depending upon the direction of potentiometric unbalance. This alternating voltage derived in the transformer secondary winding H1 is well adapted for ampli- 7 having a unity power factor.

The input circuits or triodes I43 and I 44 are connected in parallel and may be traced from thecontrol electrodes. I46 and I50, which are con nected together, through theresistance 19 to the negative conductor 48, and a cathode biasing resistance I53 to the cathodes I41 and I which also are connected together.

Anode voltage is supplied the output circuits of triodes I43 and I44 from the transformer secondary winding no. The anode of the triode m is connected to the right end terminal of the .winding I40 and the anode of the triode I44 is connected to the left end terminal of the winding I40. The cathodes of the triodes I43 and I44 are connected through the biasing resistance I53 and phase winding I21 of motor I I2.

' The motor I I2 is preferably so constructed that the impedance of. the winding I21 is of the proper value to match the impedance of the anodecircuits of the triodes I43 and I44 when the motor,

is operating in order to obtain .the most eflicient operation. Preferably, the motor is so constructed that it has a high ratio of inductance to resistance; ,for example, of the order of 6-1 or 8-1 at the frequency of the energizing current supplied to it. This provides for maximum power during the running condition of the motor with the least amount of heating, and also provides a low impedance path for braking purposes;

As noted hereinbefore, energizing current is supplied 'to the motor winding I26 from the alternating current supply source I5 through the condenser I24, The condenser I24 is so selected with respect to the inductance of the motor winding I26 as to provide a series resonant circuit By .virtue of the series resonant circuit, the total impedance of the motor winding I26 is substantially equal to the resistance of the winding, and since this resistance is relatively low, alarge current'flow through the winding I26 is made possible, This permits. the attainment of maximum power and torque from the motor II2. Furthermore, the current flow through the motor winding I26 is in former secondary winding I40, the anode of the triode I43 is rendered positive with respect to the center tap I56 while the anode of triode I44 is rendered negative with respect to said tap. During the second half cycle the anode of the triode I44 is rendered positive with respect to thetap and the anode of triode I43 is rendered negative .with respect to said tap. Accordingly, the triodes I43 and I44 are arranged to conduct on alternate half cycles of the alternating current supplied by the supply source I5.

- When no alternating signal is impressed upon the control grids I46 and I50 of the triodes I43 and I44, pulsating unidirectional current of twice the frequency of the alternating voltage supplied by source I5 is impressed onthe motor winding I21 .When thus energized, the motor H2 is not urged to rotation in either direction but remains stationary' Due to the relatively high direct current component of the current y then flowing through the motor winding I21, the

I if the rotor core structure of the motor I I2 tends to become saturated whereby the inductive reactance of the motor winding I21 is relatively small. The condenser I51 in shunt to winding I21 is so chosen that the condenser and motor winding then provide a parallel resonant circuit. This saturation of the core structure of the motor II2 operates to exert an appreciable damping effect on thej rotor I25, orin other words,an effect tending to prevent rotation of the rotor I25. Consequently, I25 hasbeen rotating, saturation of the motor core structure operates to quickly stop the rotation.

An additional effect also obtained is the introduction of a tremor into the mechanism con nected torotor I25, which tremor serves to keep that mechanism in a nascent state and thus overcomes anyadverse effects which might otherwise be encountered due to static friction of said mechanism. This tremor results from the application of pulsating current of twice the frequency of supply. source I5 on the motor winding I21 phase with the voltage of the alternating current I supply source I5 because of the series resonant circuit. The voltage across the motor winding I26, however, leads the current by substantially 90 because of the inductance of the winding I26;

Energizing current is supplied the motor winding I21 from the transformer secondarywinding I40 through the anode circuitsof the triode I43 and I44 through the circuitspreviously traced. A condenser I51 is connected in parallel with the motor winding I21 and is so chosen as to provide a parallel resonant circuit having a unity power ternal circuit impedance approximates the actual resistance of the winding I21, and since'this re- -sistance is relatively low, the impedance of the local circuit is also relatively low.

For the first half cycle of the a ternating voltage produced across the terminals of the trans- The relaunder the condition of motor operation under' consideration. That is to say, alternate pulses of current through winding I21 tend to' produce motor rotation in one direction and the remaining pulses of current tend to produce motor rotation in the opposite direction. Such operationimparts a tremor to the rotor I25.

When an alternating signal is impressed on the contro1 grids I46 and I50. the magnitude of the pulses of current flowing in the anode circuit of one triode I43 or I44 will be increased while the magnitude of the pulses of current flowing in the anode circuit of the" other triode will be decreased. Accordingly, the pulses of unidirectional current supplied to the motor .winding I21 during the first half cycle. will predominate over those supplied the motor winding during the second half cycle. Which anode current will be increased depends upon whether the alternating signal is in phase with or 180 out of phase with the voltage of source I5.

Such energization of the motor winding I21 operates to introduce therein an alternating-component of current of the same frequency as that supplied by the alternating supply source I5. This alternating component of current will either lag by 0r lead by 90 the alternating current flowing through the motor winding I26 depending upon which of the triodes I43 and I44 has its anode current increased by the prevailing alternatin signal on the control grids, and with either phase relation the two currents which the thermocouple l produce a magnetic field in the motor core structure which rotates in one direction or the other, depending upon said current phase relation and effects rotation of the. motor in the corresponding direction. Moreover, when the motor wind: ing I21 is so energized, the direct current component of the current flowing therein is decreased, and consequently, the saturation of the motor core structure is decreased with the result that the rotor damping effect is reduced and the tremor referred to disappears.

This general arrangement for operatin a reversible rotating field motor is disclosed and claimed in a copending application Serial No. 421,173 which was filed by Walter P. Wills on December 1, 1941 and which issued on July 8, 1947 as Patent No. 2,423,540.

The inherent damping characteristic of the motcr H2 contr-ributes appreciably toward the elimination. of the tor while accom balancing opera the feed bac's: cult in and 80 of my ir ention po tainment of this desired. result while ting the potentiometric rebalancing operations to be effected at extremely high speed.

With the modification of Fig. 2, similarly to the arrangement of Fig. l, the amplification of the pentode 40 is a maximum when the potentiometric network I03 is balanced or is first unbalanced so that the full unbalanced effect is initially utilized to cause energization of the motor I I2 for rotation in one direction or the other. Thereafter, when a unidirectional potential is produced across resistance 41 and is impressed on the input circuit of the pentode 40, the amplification of the pentode 40 is decreased by an amount corresponding to the magnitude of the potentiometric unbalanced potential. This decrease in amplification of pentode 40 has the same eflect on the energizing current flow to the motor winding I21 as a predetermined adjustment of contact H0 toward the new position of balance and makes possible the phenomena of motor deenergization before the contact IIO has reached the true position of rebalance. Here also, as in the arrangement of Fig, 1, this anticipating action is accentuated due to the tendency of the condenser 90 to resist any change in potential between its terminals and to maintain a disproportionately large potential thereacross as the contact IIO approaches the new balance position. As a. result, a disproportionately large unidirectional bias potential is impressed on the input circuit of pentode 40 as the contact IIO nears the new balance point to ensure that the motor H2 is deenergized for rotation the proper time before the balance point is reached as is required to compensate for the tendency of the motor H2 and its associated drive mechanism to overrun or coast beyond the true balance point.

As those skilled in the art will recognize, the reversible electrical motor II2 may be employed to operate a valve I58 positioned in a fuel supply pipe I59 for varying the supply of heating agent to the furnace I03 to the temperature of I04 is responsive, or preferably, a separate reversible electrical motor may beso employed. For example, a reversible electrical motor I60 having two opposed field windings (not shown) may be used for this purpose. The reversible motor I60 is mechanically connected in any suitable manner to the valve I50 and is adapted toadjust the latter to acting tendency of the 16 its opened and closed positions depending upon the direction to which the motor I60 is energized for rotation The mechanical connection of the motor I60 to the valve I56 is such as to increase and decrease the suppl of heating agent to the furnace as the temperature of the latter fallsbelow and rises above a predetermined level.

The motor is energized for rotation in one direction or the other, depending upon which of the two opposed field windings is energized, by means of a switch I6I. As shown, current flows from the alternating current supply source I5 through a conductor I62 to a switch arm I63 which is insulated from but is carried by the nut III which carries the potentiometer slidewire contact IIO, thence by either of the two opposed contacts I64 or I65, conductors I66 or I61 and one field winding or the other field winding of the motor I60 to the other terminal of the supply source I5. Although not shown, the contacts 464 and I of the switch I6I are made adjustable so that both the control point setting andsensitivity of the apparatus may be set in a manner well known in the art.

It will be understood that, if desired, the motor drive arrangement and control system of Fig. 2 may be employed in lieu of the relay drive system of Fig. l for effecting the desired adjustments in position of fuel valve I in response to changes in the temperature adjacent the resistance thermometer 3.

In Fig. 3 I have illustrated, more or less diagrammatically, another embodiment of my invention in which the adjustments of fuel valve I58 are effected by reversible electrical motor II2 through the medium of suitable gear mechanism I68 and a rack I 69. Fig. 3 also illustrates an alternative arrangement for neutralizing the tendency of the motor H2 and its associated mechanism to coast beyond the new balanced position of the system.

Specifically, there is illustrated in Fig. 3 a temperature control system comprising a furnace I 03 which is supplied with fuel through a pipe I50. Valve I50 is provided in the pipe I59 for controlling the flow of fuel to the furnace. The temperature of furnace I03 in this modification is indicated by a pyrometer I10 which may be of any suitable type for indicating the electromotive force developed by the thermocouple I04. For example, the pyrometer I10 may take the form of the mechanical or continuous balance" type potentiometer control instruments manufactured and sold commercially by The Brown Instrument Company, the assignee of the present invention. The Brown mechanical type potentiometer control instrument is generally disclosed in Patent 1,898,124 which was issued on February 21, 1933, to Thomas R. Harrison while the Brown continuous balance potentiometer control instrument is disclosed in the aforementioned Wills Patent 2,423,540.

Regardless of the type of potentiometer control instrument I10 employed, however, the operation of the motorI I2 is controlled in accordance with the indication of the pyrometer to control the opening of the valve I50 as is required to maintain the temperature of the furnace I03 at the desired value. To this end, provisions are made for causing the opening of the valve I56 to be proportional to the displacement of the movable element I1I of the pyrometer I10.

The movements of the movable element I1I are transmitted to the fuel valve I58 by an arrangement including an electrical control network I12.

ever, the potential between contact I" 17 The control network I12 comprises an alternating current potentiometer and includes two branches. A slidewire resistance I13 is connected in one of the branches and a slidewire resistance I 14 is connected in the other of the branches. Resistances I15 and I16 are also connected in the first mentioned branch, being disposed on opposite sides of resistance I13. The right end terminals of the two branches are connected by a conductor I11 to ground indicated at G. Alternating energizing current is supplied the control network I12 from the secondary windingI18 of a transformer I84 having a primary winding I4I connected to the alternating current supply source I5.

In accordance with this embodiment of my invention the magnitude of the alternating potential produced between ground G and the slide contact arm I1I is compared to the magnitude of the alternating potential produced between ground and the slide contact arm I19. The slide contact arm I1I is disposed in engagement with the slidewire resistance I13 and the slide contact arm I19 is disposed in engagement with the slidewire resistance I14. When the potential of the slide contact arm IN is exactly the same as that of the contact I19, the control network I12 is in a balanced condition and the reversible motor II2 will then be at rest. -Upon change in the position of contact I1I along resistance I13, howand ground will be rendered different from that between contact I19 and ground. This difference in potential is detected by an electronic amplifier arrangement to be described and is employed to control the selective actuation of motor I I2 for rotation in the proper direction to cause such adjustment of the contact I19 as is required to equalize the two potentials.

The electronic amplifier referred to includes provisions which are different from those provided in the Figs. 1 and 2 forms of my invention for anticipating the adjustment of the contact I19 to its new balance position to thereby positively insure exact neutralization of the coasting tendency of the motor and the mechanism driven thereby. As is pointed out in detail hereinafter,

the anti-hunting provisions of Fig. 3 are capable of operating in such manner as to cause momentary energization of the motor I I2 for rotation in the reverse direction when the contact I 19 approaches closely to its new balance position to thereby quickly and positively bring the motor to a stop with the contact I1'9 exactly at the new balance point. By means of this novel arrangement the motor II2 may be very fast in its rebalancing operation of the control network I12 without overshooting and consequent hunting occurring.

Specifically, the alternating potential established between the contact HI and ground G is impressed by conductors I11 and I88 on the primary winding I8I of a transformer I82 having a secondary winding I83. The transformer secondary winding I83 is connected to the input circuit of a preliminary amplifying stage 8'. Amplifying stage 6' in Fig. 3 is comprised of a triode 53 having an anode 54, a control grid 55', a cathode 44' and a heater filament 44'a. Heater filament ,44a is connected to and receives energizing current from the transformer secondary winding I9.

The input circuit of triode 53 may be traced from the control electrode 55 to one end terminal of the transformer secondary winding I83,

I cathode 44'.

. 18 through the latter, a resistance I84 to the grounded conductor 48, and a biasing resistance I85 which is shunted by a condenser I86 to the lished across the resistance I84 in opposition to the alternating potential induced in the transformer secondary winding I83 by means including an electronic valve I81, the latter of which in turn is controlled in accordance with the position of the contact I19 along the slidewire resistance I14. Thus, the resultant of the alternating potential induced in the transformer secondary winding I83 and the alternating potential produced across the resistance I84 is impressed on the input circuitof the triode 53. When the contact I19 is at rest and the potential thereof is exactly the same as the potential of the contact I, the alternating potential induced in transformer secondary winding I83 is exactly equal to and 180 out of phase with the alternating potential produced across resistance I84. Consequently, the resultant alternating potential impressed on the input circuit of the'triode 537 will be zero in value when the control network I12 is in a balanced condition.

When the alternating potential induced in the transformer secondary winding I83 is changed in response to a change in position of contact "I along the slidewire resistance I13, the alternating potential produced across the resistance I84 is changed a corresponding amount in the corresponding direction in response to adjustment of the contact I19 along resistance I14 by the motor H2. The change in potential across resistance I84, moreover, is effected in such manner as to anticipate the approach of the contact I19 to the new balance position so as to compensate for the tendency of the motor I I2 and the mechanism driven thereby to coast beyond that balance position.

The output circuit of the triode 53' is coupled by means of a condenser 18 to the input circuit of a motor driving stage 1' and operates to control the latter as in the arrangement of Fig. 2 to selectively cause energization of the motor II2 for rotation in one direction or the other depending upon the phase of the alternating current output of the triode 53.

The manner in which an alternating potential is established across the-resistance I84 under control of the contact I19 and in opposition to that induced in the transformer secondary winding I83 will now be described. This result is accomplished by impressing the potential produced across slidewire resistance I14 between ground G and the contact I19 on one of the input circuits of the electronic valve I 81 and by impressing a unidirectional potential on another of the input circuits of the valve I81 so as to control the amplification or gain of the latter in accordance with the motion and direction of motion of the contact I19 along the slidewire resistance I14.

The electronic valve I81 may be a pentagrid converter type and by way of example may be of the type known and sold commercially as the SL7. Valve I81 includes an anode I88, suppressor grid I89, two screen grids I98 and I9I which as shown are tied together, a first control grid I92, a second control grid I93, a cathode I94 and a heater filament I95. The heater filament I95 is connected to and receives energizing current from the transformer secondary winding I9.

The first mentioned input circuit of valve I81, namely that on which an alternating potential An alternating potential is estab-- 19 is impressed, may be traced from the control grid I93 through a conductor I90 to the contact I19, the siidewlre resistance I14 to the conductor I11, the grounded conductor 49 and a biasin resistance I91 shunted by a condenser I98 to the cathode I94.

Direct current energizing voltage is supplied the output circuit of the valve I81 from the posh tive conductor through a-circuit which may be traced from .the conductor 56 through a load resistance I99, the anode I88, the cathode I94 and the parallel connected resistance I91 and condenser I98 to the grounded and negative conductor 48. Direct current energizing voltage is maintained between the conductors 50 and 48 by means of a transformer secondary winding I39, rectifier valve 51 and condenser 6| as in the arrangement of Fig. 2. Screen voltage is also ob-' tained for the valve I81 from the conductors 50 and 48 through a' circuit which'may be traced from the conductor 50 through a resistance 200 to the screen grids I90 and I9I and from the latter grids through a condenser I to the grounded conductor 48. The suppressor grid I89 is internally connected to the cathode I94.

The cathode biasing resistance I91 and condenser I98 are so chosen that when the first mentioned control grid I92 is at the potential of the grounded conductor 48, the amplification factor of the valve I81 is such as to establish an alternating potential drop across resistance I84 which is exactly equal to the alternating potential induced in the transformer secondary winding I83 when the control network I12 is balanced. Thus, the amplification factor, and therefore, the gain of valve I81 is the proper value when no direct current bias potential is impressed on the control grid I92 to cause the resultant of the two alternating potentials in the input circuit of the triode 53' to be zero.

The amplification factor or gain of the valve I81 is adapted to be controlled in accordance with the motion and direction of motion of the contact I19 along the slidewire resistance I14, however, and an electrical network 202 is provided for establishing a unidirectional potential drop across a resistance 203 in accordance with the direction and extent of motion of the contact I19 along slidewire resistance I14. The resistance 203 is connected in the circuit which may be traced from the control grid I02, a contact 204 disposed in engagement with the resistance 203, through the latter to the grounded conductor 48, and the parallel resistance I91 and condenser I98 to the cathode I94.

The control network 202 for so establishing a unidirectional potential drop across resistance 203 includes a half wave rectifier 205, a resistance 206 which is shunted by a condenser 201 and both sistance 203 in series with a condenser 208. The

half wave rectifier 205 includes an anode 209, a

cathode 2I0 and a heater filament 2II which is connected to and receives energizing current from the transformer secondar winding I9.

The rectifier 205 is connected in a circuit which may be traced from the contact I 19 through conductor I96 to a conductor 2I2, the anode 209 of rectifier 205, the cathode 2I0, the parallel connected resistance 206 and condenser 201 to the grounded conductor 48 and through the conductor I11 to the right end terminal of the resistance I14. The rectifier 205, therefore, operates to establish a unidirectional potential drop across the resistance 206 and condenser 201 which varies in magpotential drop across resistance 20! and condenser 201 is impressed on the circuit includins condenser 208 and resistance 208 and thus the condenser 208 is normally charged to the potential drop existing-across the resistance 208. In such case, no current will flow through the resistance 203, and therefore, the potential drop across that resistance will be zero. Upon motion of the contact I19 to the left along resistance I14 as seen in the drawing, however, the alternating potential impressed on the network 202 will be increased to thereby cause an increase in the unidirectional potential drop established across the resistance 200 and condenser 201. This increase in potential drop acros the latter elements will operate to cause a charging current to flow into the condenser 208 and through the resistance 203 to thereby produce a potential drop across the latter of the polarity to render the upper terminal positive with respect to the lower and grounded be greater than that across the elements 200 and 201 and therefore, the condenser 208 will discharge through the elements 206 and 201 and through the resistance 203 to produce a potential drop across the resistance 203 of the polarity rendering the lower and grounded terminal positive with respect to the upper terminal.

Thus, upon motion of the contact I19 to the left the direct current potential of th control grid I92 is rendered less negative to increase the amplification factor or gain of the valve I81, and upon motion of the contact I19 to the right, the direct current potential of the control grid I92 is rendered more negative to decrease the amplification factor or gain of the valve I81. This increase or decrease in the gain of valve I81 operates to cause a corresponding increase or decreas in the alternating potential produced across the resist- IE4 may be reduced to zero before the contact I19 has been adjusted to the exact balance point.

This anticipating efiect, accordingly, may be utilized to cause deenergization of the motor I I2 for rotation before the contact I19 has reached the new balance point, and as those skilled in the art will recognize, may be employed to create a resultant potential of the opposite polarity in the input circuit of the triode 53 to cause momentary energization of the motor II2 for rotation in the reverse direction to thereby quickly stop the latter. This additional efifect is obtained by virtue of the tendency of the condensers 201 and 208 of the network 202 to resist any change in potential drop thereacross upon change in the position of the contact I19 along the slidewire resistance I14. Therefore, as the contact I19 approaches the new balance position, the direct current potential produced across the resistance 203 21 will lag slightly behind the position of the contact I19 and, consequently, the gain of the valve I81 will be changed a disproportionately large amount relatively to the displacement of the contact I19 from the new balance point to thereby cause the alternatin potential produced across resistance I84 to be changed sufficiently to effect a reversal in the resultant of the potential drops across resistance I84 and transformer secondary windin I83. As those skilled in the art will further recognize the change in amplification factor or gain of the valve I81 which is produced upon motion of the contact I19 along the slidewire resistance I14 is proportional to the rate of adjustment of the contact I19. That is to say, the magnitude of the charging or discharging current flow through the resistance 203 is proportional to the rate of adjustment to the contact I19 along resistance I14. and therefore, the magnitude of the potential drop across resistance 203 varies in correspondence with the rate of adjustment of the contact I19. Accordingly, the anti-hunting effect which is obtained varies in accordance with the rate of rebalance of the control network I12 and operates to permit the rebalancin operation of the motor I I2 to be effected at exceedingly high speed without any overshooting of the balance point occurring. 1

While in accordance with the provisions of the statutes, I have illustrated and described the best forms of embodiment of the invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention as set forth in the appended claims, and that in some cases certain featuresof the invention may sometimes be used to advantage without a corresponding use to other features.

Having now described my invention, what I claim as new and desire to secure by Letters Patent is:

1. Control apparatus for positioning objects including means for producing an electric signal, an object positioning reversible electric motor, and an electronic amplifier for controlling the direction of rotation of said motor in accordance with a characteristic of said signal, said amplifier including a variable-mu tube and means for varying the amplification factor of said tube in accordance with the movement of said motor to minimize the hunting tendency of said motor.

'2. Control apparatus for positioning objects including means for producing an alternating voltage, an object positioning reversible electric motor, and an electronic amplifier for controlling said motor as to torque and direction of rotation according to the character of said voltage, said amplifier including a variable-mu tube and means for Varying the amplification factor of said tube in accordance with the movement of said motor to minimize the hunting tendency of said motor.

3. Control apparatus for positioning objects including means for producing an alternating voltage, an object positioning reversible electric motor, and an electronic amplifier for controlling said motor as to torque and direction of rotation in accordance with the extent and direction of departure of said voltage from a predetermined value, said amplifier including a variable-mu tube and delayed action means for varying the amplification factor of said tube in accordance with the 4. Control apparatus for positioning objects including means for producing an electric signal, an object positioning reversible electric motor, electronic circuit means having an input circuit and an output circuit for controlling the direction of rotation of said motor in accordance with a characteristic of. said signal, means to impress said signal on the input circuit of, said electronic circuit means, and-means including a variable-mu tube and means for varying the amplification factor of said tube in accordance with the operation of said motor tov establish a second signal in the input circuit of said electronic circuit means in opposition to said first mentioned signal to minimize the hunting tendency of said motor.

5. Control apparatus for positioning objects including means for producing an alternating voltage,'an object positioning reversible electric motor, electronic circuit means having an input circuit and an output circuit for controlling the direction of rotation of said motor in accordance with a characteristic of said signal, means to impress said voltage on the input circuit of said electronic circuit means, and means including a variable-mu tube and delayed action means for varying the amplification factor of said tube in accordance with the operation of said motor to produce and superimpose a second alternating voltage on the input circuit of said electronic circuit means in opposition to said first mentioned voltage to minimize the hunting tendency of said motor, said delayed action means including electrioal reactive means. I

6. Control apparatus for positioning objects including a normally stabilized electrical network having a control slidewire resistance and slide contact and a follow-up slidewire resistance and slide contact, means to adjust said control slide contact along said control slidewire resistance according to the object positioning desired, means to derive an alternating voltage from said network of magnitude determined by the position of said control slide contact along said control slidewire resistance, an object positioning reversible electrical motor means having an operative connection to said follow-up slidewire resistance and slide contact to relatively adjust the latter, means to derive a second alternating voltage from said network determined by the position of said follow-up slide contact along said follow-up slidewire resistance, electronic circuit means to regulate the operate of said reversible motor means, said electronic circuit means including electronic valve means having input terminals and output terminals, means to impress said first mentioned alternating voltage on said input terminals, means to impress an alternating voltage on said input terminals in opposition to said first mentioned alternating voltage as required to minimize hunting of said reversible motor means including a variable-mu electronic valve having output ,terminals which are coupled to the input terminals of said electronic valve means and also having input terminals, means to impress said second mentioned derived alternating voltage on the input circuit of said variable-mu electronic valve, means to impress a unidirectional potential on the input terminals of said variable-mu electronic valve to vary the gain of the latter during operation of said motor including means to rectify said second mentioned derived alternating potential to produce a unidirectional potential varying in accordance with the magnitude of said second mentioned derived alternating potential, a react-- electronic valve means to energize said reversible 5 motor means for rotation in one direction or the other when the opposed alternating potentials impressed on the input terminals of said electronic valve means are unequal.

WILLIAM L. SHAFFER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,040,014 Moseley May 5, 1936 2,008,505 Jobst July 16, 1935 1,998,091 McNally Apr, 16, 1935 

